a) Field of the Invention
The invention is directed to a method for the analysis of retinal vessels based on digital images in which a ring-shaped measurement zone containing vessel portions which proceed from a central vessel and are to be measured and which differ in type between arteries and veins is arranged around the papilla, a vessel diameter being determined for each of these vessel portions, and parameters are calculated from the determined vessel diameters of the vessel portions.
The invention is further directed to an apparatus for the analysis of retinal vessels based on digital images containing a mydriatic or nonmydriatic retinal camera for recording images, a controlling and evaluating computer provided with data input media and data output media and having a system that acquires, stores and archives correlated image data and patient data, means for determining a ring-shaped measurement zone surrounding the papilla in the images which contains vessel portions which proceed from a central vessel and which are to be measured, a measuring device for determining the vessel diameter, and calculating means for determining parameters from the measured vessel diameters.
b) Description of the Related Art
By means of a quantitative vessel analysis using digital images of the fundus, it is possible to assess vascular risk which can be determined, e.g., by means of an arteriole-to-venule ratio according to Hubbard et al. (Ophthalmology, Vol. 106, December 1999, pages 2269-2280) as A/V(PSH) ratio. For example, in a study (ARIC study) it was possible to determine a relationship between the A/V ratio and the cardiovascular risk in women and for stroke and diabetes in women and men independent from blood pressure and other risk factors.
The digital fundus images are evaluated in that the A/V(PSH) ratio is calculated as a parameter by a predetermined formula from vessel diameters of arteries and veins measuring above a limiting value which are located within a ring-shaped measurement zone around the papilla. For this purpose, a central arteriolar equivalent CAE(PSH) and a central venular equivalent CVE(PSH) are calculated from the vessel diameters of all of the measured arteries and all of the measured veins based on Hubbard's formula (Ophthalmology, Vol. 106, December 1999, page 2272). The A/V(PSH) ratio is determined as a quotient: CAE(PSH)/CVE(PSH).
Wong et al. (Ophthalmology, Vol. 111, June 2004, 1183-1190) suggest selecting a small piece of a large vessel portion from the vessels identified as arteries and veins within the measurement zone and determining a mean vessel diameter characterizing the entire vessel from five measurements per vessel piece to include in the calculation of the A/V(PSH) ratio.
Because of the high measurement uncertainty with a method-related standard deviation over a number of classification limits in determining the parameters and because of the high inter-individual variability, individual follow-ups and the comparison of an individual A/V(PSH) ratio to a healthy test group have only a limited ability to discriminate between healthy individuals and those with an increased stroke risk and consequently are less suitable for individual diagnoses. In addition, the assessment of follow-ups and comparison to healthy measurements in the known method described by Wong et al. is time-consuming. Further, there is a high incidence of subjective systematic and random errors due to the manual and semi-manual implementation of the method.